The present invention is directed toward methods and apparatuses for protecting acoustically treated aircraft inlets from ice formation.
Many commercial jet aircraft are subject to governmental regulations that limit the permissible noise levels generated by the aircraft near airports. One source of noise from jet aircraft is engine noise that propagates forward from the engine through the air intake or inlet. One method for attenuating inlet noise is to line the inlet with an acoustic liner that includes a honeycomb core sandwiched between a perforated front sheet and a solid back sheet. Accordingly, each cell of the honeycomb core has an opening at the front sheet and defines a Helmholtz resonator. The perforated front sheet is aligned with the inlet flow so that sound waves in the inlet pass through the front sheet and into the honeycomb core where they are dissipated. The acoustic liner typically extends along the inner surface of the inlet to the engine.
FIG. 9 is a partially schematic illustration of an existing engine nacelle 930 having an acoustic liner system 970 in accordance with the prior art. The nacelle 930 includes an inlet 950 that supplies air to a turbine engine 922. The nacelle 930 compresses the incoming up to a throat T, then diffuses the air upstream of a fan 923. The acoustic liner system 970 includes a first segment 970a aft of the inlet throat T and forward of the fan 923. The acoustic liner system 970 further includes four segments 970b-970e positioned in a bypass duct aft of the fan 923. At least one of these segments (such as segment 970e) can be tapered, for example, to accommodate nacelle structural elements and/or to fit within the tapered trailing edge of the nacelle 930.
Commercial jet aircraft inlets also typically include ice protection systems to restrict ice formation on the aircraft when flying in icing conditions. During such flights, ice can form at the inlet hilite H and along the inlet inner and outer surfaces. To prevent ice from accumulating in the inlet, ice protection systems are designed to prevent the ice from forming.
One type of inlet anti-icing system directs hot air from the engine against the backside of the inlet inner surface, heating the inner surface to prevent ice from forming. One problem with this system is that it may not operate effectively when the inlet is lined with an acoustic liner. For example, the honeycomb cells of the acoustic liner contain generally static air, which insulates the inlet inner surface from the hot air. This can significantly reduce the heat transfer rate to the inlet inner surface and/or increase the amount of hot air required to protect the inlet from ice formation.
An approach to addressing this drawback is to have an acoustic honeycomb core with a perforated back sheet that allows the hot air to pass through the honeycomb core and the perforated front sheet. The hot air then transpires along the inlet inner surface. U.S. Pat. No. 5,841,079 to Parente discloses such a system. However, this approach may also suffer from certain drawbacks. For example, the transpiration system may not efficiently distribute the hot air removed from the engine. Accordingly, the system may require unnecessarily large amounts of hot air to be bled from the engine, which can reduce engine thrust and overall aircraft performance. Furthermore, the distribution of the hot air passing through the acoustic liner may be altered by static and dynamic pressure gradients on the inlet inner surface caused by the inlet flow field. For example, the pressure at any point in the inlet flow field can be a function of the location in the flow field, aircraft attitude, and the engine power setting. The altered hot air distribution may reduce the efficiency with which the system operates.
The present invention is directed toward methods and apparatuses for protecting an aircraft inlet from ice formation. An apparatus in accordance with one aspect of the invention includes an external surface portion, an internal surface portion positioned inwardly of the external surface portion, and a lip surface portion extending between the external surface portion and the internal surface portion to define a hilite. At least one of the external surface portion, the lip surface portion and the internal surface portion define a flow surface having first apertures. A back surface is offset from and non-parallel with the flow surface and has second apertures. A tapered acoustic core is positioned between the back surface and the flow surface such that the first apertures are in fluid communication with the second apertures through the core. The second apertures are coupleable to a source of pressurized, heated gas to direct a quantity of the gas through the first apertures sufficient to at least restrict the formation of ice on the flow surface.
In a further aspect of the invention, the acoustic core includes a continuous segment extending from the external surface portion around the hilite to the internal surface portion. In another aspect of the invention, the acoustic core includes a porous intermediate surface between the flow surface and the back surface, and has a first core portion between the flow surface and the intermediate surface, and a second core portion between the intermediate surface and the back surface. In yet another aspect of the invention, the second apertures are positioned only in a region at or forward of a minimum flow area of the inlet. In still another aspect of the invention, the first apertures define a first porosity that is greater than a second porosity defined by the second apertures.
The present invention is also directed to a method for forming an ice protection system for an aircraft engine inlet. In one aspect of the invention, the method can include disposing an acoustic core between a flow surface of the inlet and a back surface of the inlet, wherein the back surface is non-parallel to the flow surface to define a tapered region between the flow surface and the back surface, and wherein the acoustic core has an at least partially tapered shape to fit in the tapered region. The method can further include forming first apertures through the flow surface and forming second apertures through the back surface. The second apertures are sized to pass a flow of pressurized heated gas through the first apertures sufficient to at least restrict ice formation on the flow surface. In one aspect of this method, the second apertures are provided in a region only at and/or forward of the minimum flow area of the inlet.